Switching of capacitor banks is a common occurrence in electric power systems. The inductive reactance of motors in home and industrial use cause less than unity power factors, which if uncorrected can increase system losses and cause voltage levels delivered to end-use customers to drop to unacceptable levels. Capacitor banks are typically switched into the electric power circuits during high levels of inductive loading, typically during the daylight and early evening hours when most people are awake and using electric power, to correct the power factor, reduce delivery losses, and boost the voltage to the end-use customers. Once the high level inductive loading subside, typically at night, the capacitor banks are switched out of the electric power circuit. Daily cyclical use of capacitors is therefore a common practice to balance the capacitive reactance with inductive loads, and thus minimizing the stated problem, as electric loads increase and decrease on a daily basis.
Because inductive residential loads typically increase and decrease on a daily cycle, capacitor switching in response to residential loads typically occurs on a daily basis. Capacitor switching can also occur multiple times daily, for example when residential loads are combined with industrial or municipal loads that occur at night or multiple times per day. Coal mining equipment, aluminum smelters, manufacturing assembly lines, municipal water pumps, and electric transportation loads, to name but a few examples, can place large, cyclical or intermittent inductive loads on an electric power system. As a result, capacitor switches often experience several hundred to several thousand operations per year. Circuit breakers that are designed to operate in response to overload and other emergency conditions, by comparison, typically operate much less frequently, on the order of only a few isolated operations up to a couple of dozen times per year.
Nevertheless, electric utilities have often utilized the same circuit switching technology used in circuit breakers for capacitor switching applications, despite the fact that the circuit breaker technology is not designed to operate nearly as frequently as capacitor switches typically experience. For example, circuit breakers are typically designed for an industry standard of 2,000 to 10,000 operations, which is intended to cover the entire lifetime of the circuit breaker. While this standard is robust and appropriate for a circuit breaker that can be expected to operate only a couple of dozen times per year, it is inadequate for a capacitor switch that can be expected to operate several hundred to several thousand operations per year. Conventional circuit breaker technology can therefore be expected to wear out too quickly when put into operation for capacitor switching applications. Circuit breakers are also designed to switch under very high short-circuit emergency current conditions, which capacitor switches are not expected to experience in normal daily operation. It is therefore inefficient and to utilize circuit breaker technology for capacitor switching applications.
As a result, capacitor switches have been designed to withstand tens of thousands of cycles. Commonly owned U.S. Pat. Nos. 7,115,828; 7,078,643; 6,583,978; 6,483,679; 6,316,742 and 6,236,010 are good examples of electric power switching technology designed specifically for the capacitor switching application. For these capacitor switches, the spring mechanism that accelerates the electrical contactor is a critical component. Because a capacitor switch normally operates to switch the capacitor bank into or out of an energized power circuit on both the opening and the closing stroke, the electrical contactor must be accelerated to an appropriate speed on both the opening and the closing strokes. In addition, because the capacitor switch is designed to cycle on at least a daily basis, the capacitor switch is preferably motorized so that it can be operated from a remote control center or automatically in response to monitored line conditions. When the capacitor switches are well designed and cost effective, an electric utility typically finds it economically feasible to install capacitor banks in many locations throughout the electric power sub-transmissions and distribution system, resulting in a dozens or hundreds of economical capacitor switch installations for a particular electric utility, and thousands of economical capacitor switch installations across the greater power grid. Considerable effort therefore goes into designing capacitor switches that have advantageous size, cost, operating and reliability characteristics.
One prior capacitor switch design in described in U.S. Pat. No. 4,636,602, which discloses a capacitor switch with a bi-directional toggle mechanism and nested opening and closing springs. However, the toggle mechanism in this design relies on expanding and contracting latch rings on a piston inside a cylinder to charge and release the opening and closing springs. Although this design is functional, the latch and trip mechanisms for the expanding and contracting latch rings are complex and can cause undesirable binding in the slide mechanism. The piston and cylinder arrangement is also a relatively large, bulky and heavy design. Placing the trip and latch components within a closed cylinder also makes it difficult to inspect and service these components, requiring disassembly of the drive unit and removal of the piston and cylinder latch mechanism.
Accordingly, there is an ongoing need for a cost effective electric power switch suitable for use as a capacitor switch. There is a further need for a capacitor switch that includes an improved bi-directional toggle mechanism that does not rely on expanding and contracting latch rings located on a piston inside a cylinder to charge and release the opening and closing springs.